Heat-Pump Water Heaters

Heat-pump water heaters produce more than twice as much hot water per kilowatt-hour (kWh) of electricity consumed as standard electric water heaters. How do they work and when do they make sense?

A heat pump relies on the well-understood refrigerant cycle to move heat from one place to another. The same basic principle used for a refrigerator can also be used to heat water. While an electric-resistance water heater (the standard electric storage water heater that is sold by the millions each year) converts electricity directly into heat, a heat pump water heater uses electricity to extract heat from the room and deliver it—at a higher temperature—into the water. In doing so, the “efficiency” jumps from somewhat less than 100% (for electric resistance water heating) to more than 200%.

Heat-pump water heaters can either be self-contained, with a heat-pump unit that’s directly integrated with the storage tank, or modular, with an add-on heat-pump unit that connects to a standard storage-type water heater. In either case, a fan pulls air from the room into a heat exchanger core where its heat is transferred to refrigerant in copper coils—this occurs in the evaporator coil. The refrigerant is colder than the circulating air, so heat is transferred to the refrigerant, evaporating it. A compressor mechanically compresses the refrigerant gas, raising its temperature. The warm refrigerant gas then passes through a condensing coil, where it is condensed by the (cooler) circulating water from the storage tank, heating the water in the process. The refrigerant liquid then passes through an expansion valve where the temperature drops and the cooler refrigerant enters the evaperator coil to repeat the cycle.

Energy performance of heat-pump water heaters is measured as either the coefficient of performance (COP) or Energy Factor. The COP, used for modular heat-pump water heaters, is a simple measure of instantaneous efficiency—a COP of 2.5 means an efficiency of 250%. Energy Factor is used for integral heat-pump water heaters, accounting for storage losses, and is a more accurate measure of actual performance.

Self-contained heat-pump water heaters are made by a number of companies, including Rheem, GE, and Stiebel-Eltron. Add-on heat pump units that attach to conventional storage-type water heaters are made by A.O. Smith, AirGenerate, and North Road Technologies.

A heat-pump water heater extracts heat from the space where it is installed—cooling the room—so this should be considered when determining placement. If it’s installed in conditioned space, your heating system is going to use more fuel, and that effectively reduces the energy performance of the heat-pump water heater. During summer, the cooling and dehumidification of room air is an advantage—providing free air conditioning.

Heat-pump water heaters have fans and pumps, which will generate some noise—similar to a refrigerator. Unlike standard water heaters, they also require condensate drains to remove moisture that condenses on the evaporator coil.

December 1, 2010

DISCUSSIONS

Reader-contributed comments related to Heat-Pump Water Heaters - EBN: 19:12. Comments are listed with newest at the top.

Heat-Pump Water Heaters Posted by Bruce Donelson on Nov 30, 2010, 07:47 PM
These would be fine in a cooling-dominated climate. For heating climates, the additional coolth delivered inside the building envelope would detract from their actual efficiency. None of the available units is set up to duct the waste coolth to the outside of the building envelope when that would be the preferable option, and none of them have published their efficienies if they are using cold (unconditioned) air.
Bruce

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